1. Field of the Invention
This invention relates to a subterranean well tool, such as a bridge plug, for temporary installation in an oil well conduit, of the type having an elastomeric seal element producing relative movement between cooperating cone and slip elements to sealingly lock the bridge plug in the conduit, wherein pressure-equalization across the elastromeric seal element is effected prior to releasing the slip elements from engagement with the conduit.
2. Description of the Prior Art
Bridge plugs have been employed in subterranean wells for many years. The primary purpose of a bridge plug is to provide a temporary anchoring of a tool at a desired location in a well conduit, accompanied by a sealing of the annular space between the tool and the internal wall of the well conduit. Bridge plugs in commercial use, such as the Model C Retrievable Bridge Plug sold by BAKER SERVICE TOOLS of Houston, Tex., typically utilize a hollow tubular body on which two axially spaced sets of cone elements are secured. Intermediate the cone elements, a pair of axially spaced sets of slip elements are mounted on a cross-link which is supported in diametrically opposed slots in the tubular body so that relative movement of the tubular body with respect to the slip elements in either direction will produce an outward camming of one set of slips into engagement with the conduit wall, but not the other set of slips. Such relative movement is produced by one of a pair of annular elastromeric sealing elements, sometimes referred to as packer cups, which are respectively mounted on the tubular body adjacent the cone elements. One packer cup is responsive to a downward fluid pressure differential to move the tubular body downward, while the other is responsive to an upward fluid pressure differential to move the tubular body upwardly. Thus, when such a conventional bridge plug is positioned at a desired location in the well conduit, the existence of an upwardly or downwardly directed fluid pressure differential across the packer cups will determine whether the lowermost cone moves upwardly to engage the adjacent set of slips or the uppermost set of cones moves downwardly to engage the adjacent set of slips. In either event, only one set of slips is cammed outwardly into engagement with the conduit wall in response to the existing fluid pressure differential.
The tubular body is carried into the well by a mandrel inserted therethrough which is provided with an appropriate slot to accommodate the cross-link. A running-in tool engages the upper end of the mandrel and the upper end of the tubular body to maintain these elements in a desired axial relationship during the running in of the tools. An annular fluid passage is defined between the exterior of the mandrel and the interior of the tubular body and such passages are closed at each end by a spring pressed annular piston valve. A downwardly facing abutment shoulder on the mandrel is engagable with the lowermost piston valve to shift such valve to an open position by downward movement of the mandrel as the first step in releasing the bridge plug from the conduit. The opening of the lower piston valve produces an equalization of pressure across the lowermost elastromeric sealing element so that the slip element associated with the lowermost cone may be moved upwardly off the cone surface by upward movement of the tubular body member. Such upward movement of the tubular body member is produced by a shoulder provided on the mandrel which engages the uppermost valve and moves it to an open position and then into abutting relationship with a shoulder on the tubular body to initiate the upward movement of the tubular body. Thus, with both valves having been successively opened, it was presumed that pressure balancing across the two elastomeric seal elements would be achieved.
In the practical utilization of this type of valve, some difficulties have been encountered. For example, the initial downward movement of the mandrel required to open the lower piston valve to achieve equalization of pressure across the lower elastomeric sealing element is often accomplished by the operator very rapidly, so the pressure differential across the lower elastomeric sealing element is not completely neutralized. As the mandrel is moved upwardly, any residual pressure differential or pressured well fluids will cause the lower cone to remain in engagement with the lower set of slips and effect the anchoring of the tubular body to the conduit wall, thereby preventing withdrawal of the tool without incurring serious damage to the conduit wall and/or the slips. Also, as the tool is raised upwardly through the well conduit, it may pass through production zones where well fluids are flowing into the conduit, thus providing a fluid pressure differential across the lower elastromeric sealing element with the same adverse results as heretofore mentioned.
In an effort to overcome these difficulties, another type of tool has been developed which is disclosed in co-pending application Ser. No. 417,012, filed Sept. 13, 1982, now U.S. Pat. No. 4,460,041 and assigned to the Assignee of this application. This tool employs a bypass fluid passage within the control mandrel. Three sleeve-type sliding valves are then provided to cooperate with the control mandrel so the downward movement of the control mandrel establishes a bypass fluid passage around the lowermost one of the annular elastomeric sealing elements and upward movement of the mandrel establishes a permanent fluid bypass around both annular sealing elements which remain in effect after the control mandrel engages the body of the tool to withdraw same from the well conduit. The provision of the fluid bypass within the interior of the control mandrel might have a tendency to weaken the physical strength of this element and, of course, might make the control mandrel somewhat more expensive to manufacture. Additionally, the flow area available for the bypass passage is somewhat restricted and, obviously, as large as possible bypass flow area is desired in order to assure complete neutralization of any fluid pressure differential across either of the annular elastomeric sealing elements as either upward or downward movement of the tubular body carrying such elements is effected by the control mandrel.